Because capillary permeability is constant under normal conditions, the filtration coefficient can be used to determine the relative number of open capillaries (i.e., the capillary surface area available for filtration in tissue). For example, the increased metabolic activity of contracting skeletal muscle relaxes the precapillary resistance vessels and hence opens more capillaries. This process, called capillary recruitment, increases the filtering surface area.
Disturbances in Hydrostatic-Osmotic Balance. Relatively small changes in Pa may have little effect on filtration. The change in pressure may be countered by adjustments in precapillary resistance vessels (autoregulation; see ) so that hydrostatic pressure remains constant in the open capillaries. However, a severe reduction in Pa usually evokes arteriolar constriction mediated by the sympathetic nervous system. This response may occur in hemorrhage, and it is often accompanied by a fall in Pv. These changes reduce capillary hydrostatic pressure. However, the lowering of blood pressure in hemorrhage causes a decrease in blood flow (and hence in O2 supply) to the tissue, with the result that vasodilator metabolites accumulate and relax the arterioles. Precapillary vessel relaxation also occurs because of the reduced transmural pressure (autoregulation; see ). Consequently, absorption predominates over filtration, and fluid moves from the interstitium into the capillary. These responses to hemorrhage constitute one of the compensatory mechanisms used by the body to restore blood volume (see
An increase in Pv alone, as occurs in the feet when a person stands up, would elevate capillary pressure and enhance filtration. However, the increase in transmural pressure closes precapillary vessels (myogenic mechanism; see ), and hence the capillary filtration coefficient actually decreases. This reduction in capillary surface available for filtration prevents large amounts of fluid from leaving the capillaries and entering the interstitial space.
In a healthy individual, the filtration coefficient (kt) for the whole body is approximately 0.006 mL/minute/100 g of tissue/mm Hg. For a 70-kg man, an elevation in Pv of 10 mm Hg for 10 minutes would increase filtration from capillaries by 420 mL. Edema does not usually occur because the fluid is returned to the vascular compartment by the lymphatic vessels. When edema develops, it usually appears in the dependent parts of the body, where the hydrostatic pressure is greatest, but its location and magnitude are also determined by the type of tissue. Loose tissues, such as the subcutaneous tissue around the eyes or in the scrotum, are more prone than firm tissues, as in a muscle, or encapsulated structures, as in a kidney, to collect larger quantities of interstitial fluid.
Pinocytosis. Some transfer of substances across the capillary wall can occur in tiny pinocytotic vesicles. These vesicles (see
Figs. 17.26 17.27 ), formed by the pinching off of the endothelial cell membrane, can take up substances on one side of the capillary wall, move them across the cell by kinetic energy, and deposit their contents on the other side. This process is termed transcytosis. The amount of material transported in this way is very small in relation to that moved by diffusion. However, pinocytosis may be responsible for the movement of large (30-nm) lipid-insoluble molecules between blood and interstitial fluid. The number of pinocytotic vesicles in endothelium varies among tissues (amount in muscle > amount in lung > amount in brain), and the number increases from the arterial end to the venous end of the capillary.